1 Field of the invention.
The present invention relates to a process for the manufacture of an insert. More particularly, the present invention relates to a process for the manufacture of an insert from powdered metal which is subsequently sintered.
2 Information disclosure statement.
The present invention is particularly but not exclusively applicable to replaceable inserts for a cutoff tool of a screw machine.
In a typical screw machine, a cutoff tool moves radially relative to a rotating workpiece for performing parting and grooving operations relative to the workpiece.
More particularly, as the workpiece is rotated and the cutoff tool is brought into engagement with the rotating surface of the workpiece, the rotating surface rotates at a relatively high surface speed relative to the cutoff tool. However, although the rotational speed of the workpiece remains constant, the surface speed gradually decreases as the cutoff tool moves radially towards the axis of rotation of the workpiece.
Although cemented carbide or cermet inserts have been proposed, such carbide inserts tend to chip and break when used at the aforementioned relatively low surface speeds encountered in cutoff operations.
Such low surface speeds which may be in the order of 0.10 to 250 ft./min. require the provision of a high speed steel blade. Such high speed steel blades have been successfully applied at any speed as they have the required toughness characteristics to withstand low surface speed cutoff operations without breaking or chipping.
Nevertheless, high speed steel blades require frequent regrinding which is time consuming and labor intensive.
The present invention provides a replaceable insert and a method of manufacture thereof which is tough enough to withstand the rigors of low surface speed cutoff operations while combining the advantages of being readily replaceable as an insert within a suitable holder.
Furthermore, the insert of the present invention may have a cutting width of as little as 0.025 inches while maintaining the required toughness to withstand chipping and breakage of the insert at very low surface speeds.
Those skilled in the art will readily appreciate that by use of a cutting width of as low as 0.025 inches, the amount of metal removed during the cutoff operation is minimal thereby resulting in a corresponding saving of materials.
The present invention overcomes the aforementioned problems associated with high speed steel tools which require periodic regrinding. Also, the present invention provides a process for the manufacture of an insert having a microstructure exhibiting a uniform distribution of carbides throughout. Additionally, the present invention overcomes the problem of segregation during the molding process.
An article by Ralph W. Stevenson entitled "Powder metallurgy Tool Steels" describes the Fulden process for manufacturing replaceable inserts having improved properties and product performance. The Fulden process provides an insert having nearly 100% full density.
The Fulden process essentially includes the steps of compacting powdered metal and subsequently heating the compact within a sintering furnace in order to achieve nearly 100% full density within the resultant insert.
In the present invention the unexpected discovery was made that by heating the compact to a first temperature just above the melting point of the powdered metal until a first portion of the powdered metal melts and then rapidly cooling the compact to a few degrees below the melting point, densification of a first portion of the powdered metal in the solid phase takes place. Furthermore, by the process according to the present invention, the profile integrity of the compact is maintained and the resultant insert can be reproduced to within an extremely narrow tolerance.
Also, an insert having a cutting width within the range 0.025 to 0.500 inches can be manufactured according to the present invention, the insert having the necessary toughness for low speed applications.
Additionally, inserts produced according to the present invention may be coated because no grinding of the resultant insert is required whereas coating the prior art high speed steel tools was redundant because such coating would be removed during regrinding.
The inserts according to the present invention are particularly suitable for low speed cutoff and grooving operations on a screw machine.
In view of the reproducibility to within a close tolerance, the insert according to the present invention enables automatic precision machining to be restored subsequent to insert replacement without the need for grinding or other adjustments.
The present invention includes sintering the compressed powdered metal in an evacuated furnace until 60 to 80% of the insert is in the liquid phase and then reducing the temperature in order to achieve densification of the remaining 20 to 40% of the insert that is in the solid phase. The resultant insert after cooling thereof has improved properties particularly for low surface speed operations.
The present invention therefor provides an improved process that overcomes the problems associated with the prior art high speed steel tools that require grinding and provides an insert and method of manufacture thereof that makes a considerable contribution to the art of manufacturing cutting tools.
Another object of the present invention is the provision of a process that provides an insert for low surface speed application while avoiding the inconvenience of grinding.
Another object of the present invention is the provision of a process that provides an insert that does not require time consuming grinding thereof and which can be manufactured to within a narrow tolerance.
Another object of the present invention is the provision of a process that provides an insert which maintains the profile integrity of the compact from which the insert is formed.
Another object of the present invention is the provision of a process that provides a narrow insert thereby reducing the amount of waste metal removed from the workpiece during cutoff operations.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings